Ozarcus enters the LRT as a basal stem bony fish (and a human ancestor)

Following the advent of transverse toothless jaws
in Silurian Loganiella and its extant sisters, Rhincodon (the whale shark) and Manta, the Large Reptile Tree (subset Fig. 1) splits taxa with newly acquired anteroposteriorly deeper jaws + marginal teeth in two during the first great dichotomy. At this point the LRT splits elasmobranchii (sharks + ratfish in blue Fig. 1) from stem bony fish like Ozarcus (Figs. 2, 3).

In phase two,
The clade of stem bony fish (that had no bone) quickly and ultimately evolved into genuine bony fish like Amia (the bowfin), at the top of the orange column (Fig. 1) and Trachinocephalus (the lizardfish), at the top of the yellow cladogram (Fig. 1).

Members of this clade and those that directly precede it
are step-wise ancestors to tetrapods, mammals and humans.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure 1. Updated subset of the LRT, focusing on basal vertebrates = fish.

The basalmost stem bony fish in the LRT
is now Ozarcus here (Figs. 2, 3) shown for the first time with bones colored with tetrapod homologs for scoring.  It had large eyes, a deep jawline and the tiniest marginal teeth. Such teeth were a new invention and this is how they started, not much bigger than dermal denticles. It was so close to sharks, it was originally described as “a Palaeozoic shark with osteichthyan-like branchial arches.”

With its giant eyes and short rostrum
Ozarcus also looked like a spiny shark (clade: Acanthodii), but Ozarcus nests in a different node (Fig. 1) and shows how jaws developed from anterior gill arches. Despite the Early Carboniferous appearance of Ozarcus in the fossil record, the genesis of this taxon goes back to the mid-Silurian based on phylogenetic bracketing.

Figure x. Skull of Ozarcus colored using DGS methods and tetrapod homologs. Note the tiny teeth inside the large tooth basin. The narrowness of the skull is partly due to crushing.

Figure 2. Skull of Ozarcus colored using DGS methods and tetrapod homologs. Note the tiny teeth inside the large tooth basin. The narrowness of the skull is partly due to crushing.

Ozarcus mapesae (Pradel et al. 2019; Early Carboniferous, 325 mya; AMNH FF20544) nests with Falcatus in the LRT, between Loganiella and Doliodus. The authors considered the palatoquadrate (pq) a single bone (Fig. 3) rather than the jugal, pterygoid, ectopterygoid, quadrate and lacrimal shown here (Fig. 2). The intertemporal anchors the hyomandibular here (eh) as in other vertebrates. The teeth are extremely tiny, much smaller than each tooth basin in the maxilla. The premaxilla is a tiny area between the ventrally opening incurrent nares. A ghosted prefrontal is shown framing the excurrent naris. The seemingly crushed narrowness of the skull was likely broader in life, especially around the gill basket.

FIgure 1. Ozarcus from Pradel et al. 2019 animated to show the mandible/gill elements opening and closing.

Figure 3.  Ozarcus from Pradel et al. 2019 animated to show the mandible/gill elements opening and closing.

Sister taxa, like Falcatus
(Fig. 4; another Early Carboniferous late survivor of an earlier Silurian radiation), helps us imagine what the post-crania of Ozarcus might look like. Falcatus also had a soft rostrum made of nasal bone precursors.

Figure 3. Falcatus skull. This taxon is close to Polyodon in the LRT.

Figure 4. Falcatus skull. This taxon is close to Polyodon in the LRT.

Lacking such a rostrum is Doliodus
(Fig. 5)  from the Early Devonian. Rather than a tall narrow skull, as in Ozarcus, Doliodus had a wider skull with odd, two-pronged teeth. Such teeth represent an early experiment as toothed jaws were a new thing.

A basal elasmobranch still living with us today,
Chimaera, also has a tall narrow skull, like that of Ozarcus. Chimaera gives us a  model for restoring what the post-crania of Ozarcus might have looked like.

Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Figure 5. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Taxa in the LRT
document a gradual accumulation of derived traits. This is well illustrated with these related taxa, all of which look like one another, more so than any other included taxon. The tree topology has settled down now. New taxa drop in without upsetting the nesting of taxa already in the tree.


References
Pradel A, Maisey JG, Tafforeau P, Mapes RH and Mallant J 2014. A Palaeozoic shark with osteichthyan-like branchial arches. Nature 13185. doi:10.1038/nature13195e

wiki/Ozarcus

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